Abstract

Introduction: Essential tremor (ET) is a neurological disorder which is manifested by spontaneous rhythmic involuntary movements, which are result of alternating contraction of agonist and antagonist muscles. In most cases it affects upper limbs, but can also spread to other parts of the body, head and larynx for example. It is typically a postural tremor, but can also emerge during movements (kinetic tremor). Patients with ET have difficulties carrying out normal daytime activities: eating, drinking and speaking, some studies show evidence of prefrontal cortical dysfunction. The goal of our study was to see if there is a difference in the latency of the P300 wave between patients with ET and healthy people as well as determine eventual differences in prefrontal cognitive tests scores. Methods: Subjects: 13 patients with ET (8 females, 5 males), 8 subjects in the control group (3 males, 5 females), with similar educational level. The mean age of the patients was 63.62, in the control group 50.86. Seven patients had positive family history for ET. Tests: Digit span (forward and backward - DSF and DSB), Verbal Fluency Test, Conflicting Instructions Test, Go-No-Go test (change of instructions), Beck Depression Inventory (BDI), Minimental State Examination (MMSE). Three channel EEG was used to determine the P300 wave latency. Subjects shown to be depressed on BDI and those with a MMSE score less than 25 were excluded from the study. Results: Although the BDI scores in both groups were bellow 10 (which means that there was no clinically important level of depression), the difference between groups was statistically significant (t=3.1, p=0.05). There was no significant difference in the DSF, but the difference in the DSB was statistically significant (t=2.24, p=0.02). Two groups significantly differed in the Verbal Fluency Test (t=2.62, p=0.01) but did not show any difference in the Conflict Instruction Test and Go-No-Go task. There was no statistically significant difference in the P300 latency at any point (Fz, Cz, Pz) between the patients with ET and the control group. The amplitudes at all three points were similar. Conclusions: DSB and Verbal Fluency Test showed statistically significant difference between ET patients and control group (t=2.62, p=0.01). It could be that these two tests are sensitive enough to detect prefrontal cognitive impairment in patients with ET. The difference in the BDI scores between the two groups deserves attention as well. This could be a result of the disease per se, knowing that ET is a chronic condition, which could be socially distracting. Consequently, patients with ET show higher level of depression compared to controls.
We expected to find difference in the P300 latency between the two groups, which would have been a neurophysiological and objective evidence for the prefrontal cognitive impairments in ET patients. On contrary, the results showed no difference in the P300 latency between the two groups. It could be that the number of patients and controls in this pilot study is too low to draw conclusions, or simply P300 is not a sensitive marker of the prefrontal cognitive impairment in ET patients.